Process for reducing corrosion by ammoniacal solutions of inorganic salts and new composition of reduced corrosiveness



Patented Nov. 5, 1940 UNITED STATES PATENT OFFICE PROCESS FOR REDUCING CORROSION BY AMMIONIAUAL SOLUTIONS OF INORGANIC SALTS AND NEW COMPOSITION OF RE- DUCED CORROSIVENESS Herman A. Beekhuis, Jr., Petersburg, Va., and

Charles K. Lawrence, Baldwinsville, N. Y., assignors to- The Solvay Process Company, New York, N.- Y., a corporation-of New York No Drawing.

Application June 13, 1938 Serial No. 213,452

14 Claims.

reduce or prevent, the corrosion of apparatus constructed of ferrous metals in which these solutions are handled constitutes a major problem in the commercial use of these solutions, since, when they are shipped in tank cars of the usual construction, rapid corrosion occurs and the tank soon is not usable. Also corrosion occurs in apparatus constructed of ferrous metals in which the solutions are handled.

It is, therefore, an object of the present invention to provide a method for reducing the rate of corrosion of ferrous metals by solutions of in- .organic salts in ammoniacal liquors to an extremely low rate.

It is a further object of this invention to pro-' 5 vide solutions of inorganic salts in ammoniacal liquors which corrode ferrous metals only extremely slowly.

It isa further object of this invention to provide a. method for rendering solutions of inoro ganic salts, in ammoniacal liquors, which normallycorrode ferrous metals rapidly, much less corrosive intheir action on said ferrous metals. In general, ammoniacal solutions of inorganic salts are corrosive with respect to ferrous metals 5 and the process of this invention may be emplayed to reduce the rate of corrosion of ferrous metals by such solutions. The invention is particularly applicable toammoniacal solutions of those nitrogenous salts which are employed in the production of fertilizers; for. example, solu-' tions of ammonium salts, such as ammonium ni:

trate and ammonium chloride, or nitrates'such' assodium nitrate and ammonium nitrate; or soto an extremely low rate.

this invention to reduce the corrosion of ferrous metals contacted therewith.

According to the present invention, the apparatus, which may be a storage tank, tank car, conveying pipe, measuring tank or the like, is. 5

constructed of the ferrous metals commonly used for this purpose such as low carbon steels, for example structural steel, cast iron and wrought iron.

In employing this invention a compound .of

trivalent arsenic is incorporated in a solution of an inorganic salt in an ammoniacal liquor, such as aqua or anhydrous liquid ammonia, particularly an ammoniacal liquor of 5% or greater strength. The trivalent arsenic compound added to the ammoniacal solution '(i. e. prior to incorporation in the solution) may be, for example, arsenic trioxide (AS203) or a sulfide of trivalent arsenic, for instance, AS2S2 (realgar) or AS283, such as precipitated arsenic trisulfide or orpio ment, or an arsenite, for instance sodium or ammonium arsenite. In case arsenic trioxide is used, it may be in any of its forms, for instance white arsenic, either crystalline, as'octahedral crystals or amorphous, or vitreous arsenic trioxide. The incorporation of a trivalent arsenic compound in a solutionof an inorganic salt in an ammoniacal liquor reduces the rate of corrosion of ferrous metals in contact with the solution 80 Compounds of trivalent arsenic dissolve but slowly when added directly to the ammoniacal salt solutions except at elevated temperatures which, if employed, would necessitate the use of high pressure equipment and the consumption of large quantities of heat. Accordingly, the arsenic compounds are preferably first dissolved in an alkaline solution, such as an aqueous solution of soda ash, caustic soda, ammonia, or the like. containing arsenic may then be added to the ammoniacal salt solution.

The quantity of arsenic compound added may vary from a very small percentage upward, and

may vary with the composition of the salt solu- 5 tion. In general, it is preferred to add the arsenic compound in the amount of about 0.05% or more (calculated as AS203) The objects of the invention may be obtained by addinglarge amounts of thearsenic compounds, although the arsenic 50:

Small quantities of the alkaline solution 40 compound in solution will constitute a minor proportion by weight of the total composition and no added advantage is obtained by adding the arsenic compound in excess of the solubility thereof in the particular solution employed. Less than 0.05% may be added, though the protection afforded is not as great as with the preferred amounts.

Our invention has specific application to ammoniacal solutions of the nitrates, particularly of ammonium nitrate. It appears from our observations of the action of numerous salt solutions on ferrous metal that the chemical reaction which causes rapid corrosion of ferrous metals by ammoniacal ammonium nitrate solutions is preculiar to nitrate solutions and to ferrous metals. Further, in the case of the nitrate solutions, rapid corrosion of the metal as a result of this reaction occurs only with the ammoniacal solutions. Thus, in the case of acid and neutral solutions of ammonium nitrate, hydrogen is evolved by the corrosion of ferrous metal by the solutions. In the case of corrosion by ammoniacal ammonium nitrate solutions, instead of detecting the evolution of hydrogen, we have observed that ammonia is formed by reactions taking place during corrosion of the metal by the solution. Under the conditions encountered in the corrosion of ferrous metals by ammoniacal ammonium nitrate, the presence in the solution of even a small amount of a trivalent arsenic compound appears to inhibit the particular type of corrosion involved.

The preferred ammoniacal solutions of nitrate to which our invention has specific application are aqueous solutions of ammonium nitrate containing free ammonia in which the ammonium nitrate constitutes about 50% or more of the solution (for example, 40% to and in which the ammonium nitrate is dissolved in 5% or stronger aqua ammonia, particularly in an aqua ammonia of about 25% or greater strength due to the relatively high rates of corrosion exhibited by these solutions.

Our invention further has specific application to the prevention of corrosion by ammoniacal urea-carbonate solutions; that is, ammoniacal solutions of mixtures of urea, ammonium carbonate and ammonium carbamate such as may be produced in conjunction with the synthesis of urea. Our preferred urea-carbonate solutions, to which the invention relates, may contain between about 25% and about 50% urea, between about 5% and about 30% of combined ammonia and carbon dioxide, in the form of ammonium carbonate and ammonium carbamate, and the remainder an ammoniacal liquor of 5% or greater strength. We have found the addition of a trivalent arsenic compound, as above indicated, is very effective in reducing the rate of corrosion of ferrous metals by these solutions.

In order to protect the ferrous metals even further, we have found it desirable also to add to the solution a compound of the general class of compounds which contain divalent sulfur linked to carbon and preferably linked to an atom of carbon with the remaining valences of the carbon atom linking the carbon to nitrogen. As illustrative of these compounds which further reduce the rate of corrosion, the following are noted:

(a) Carbon disulfide (S=C=S);

(b) The thio alcohols, such as ethyl mercaptan (C2H5.SH), and benzyl mercaptan (CsHsCHaSH) (c) The thlo ethers, such as diethyl sulfide [(C2H5)2S];

(d) The organic thio acids, their salts and esters, such as:

(1) Thiocyanates (XSCEN), e. g., ethyl thiocyanate, alkali metal thiocyanate, ammonium thiocyanate, etc.

(2) Derivatives of the thio analogues of carbonic acid, such as potassium xanthate (3) Thiocarboxylic acids, such as thioacetic acid such as thiophenol and thio beta naphthol HJ and thiodiphenyl amine \NH CsH4 Compounds containing divalent sulfur linked to carbon are suitable for use in carrying out this invention, but it is preferred to use compounds in which the remaining valences of the carbon atom linked to divalent sulfur are satisfied by one or more atoms of nitrogen such as the thiocyanates and thiourea.

The quantity of organic sulfur compound which may be incorporated to reduce the rate of corrosion may vary from a very small percentage upward. It is preferred, however, that the solution contain 0.004% to 0.8% of these materials (calculated as sulfur). More than 0.8% (calculated as sulfur) may be present although the presence in the solution of a high proportion of the sulfur compounds is not necessary to accomplish the purposes of the invention and the amount of the sulfur compound is, in general a minor proportion by weight of the amount of the total inorganic salt content of the solution. For example, any of the materials listed in the preceding paragraph may be added, to a solution containing 60 parts of ammonium nitrate, 20 parts ammonia, 20 parts water and 0.1% arsenic trloxide (AS203), in amount corresponding to 0.04% sulfur.

The rate of corrosion of ferrous metals by ammoniacal salt solutions varies with the varying conditions under which the solutions and metals are in contact with each other. The protection afforded the metal under some conditions enpounds'to the solution. Further, the addition of these sulfur'compounds serves to protect the arsenic compounds in the solution. Thus, while under most conditions the presence of the arsenio compound alone affords a lasting protection against-corrosion; when corrosion does occur a product of the reaction of ferrous metals with ammoniacal solutions of inorganic salts appears to be a ferrous compound which is soluble in the solution but is oxidized by the air to an insoluble ferric compound. This ferric compound is precipitated from the solution andappears to adsorb arsenic-compounds. By incorporating in the solution a divalent sulfur compound, removalfrom I the solution of arsenic compound, with a consequent loss in the protective action of the arsenic compound in the solution against corrosionof the metal, may be prevented. Accord- I ingly, the divalent sulfur compounds perform, in conjunction with the arsenic compounds, the

double function of protecting'the metal against corrosion under varying conditions and of guarding against loss of the protective action of the trivalent arsenic compounds. 1

Example 1.-An alkaline arsenite solution is prepared by dissolving 10 parts octahedral crystalline arsenic trioxide in 90 parts of a aqueous solution of sodium hydroxide at room temperature. One part of the arsenite solution soprepared is added to 100v parts of a solution containing 60% ammonium nitrate, ammonia, and the remainder chiefly water. The solution so produced contains 0.1% arsenite (cal- 'culated as AS203).

The thus treated solution is then contacted with apparatus made of ferrous metal. It has been found that the rates of corrosion of low carbon steel and cast iron contacting the solutions with low carbon steel apparatus, similar results are obtained as in the above example. I

, Example 3.To 100 parts of a solution containing 60% ammonium nitrate, 20% ammonia, and 20% water are added 0.1 part ammonium thiocyanate and 0.5 part of an arsenite solution prepared as in Example 1. The solution, which then contains 0.04% ammonium thiocyanate (calculated as sulfur) and 0.05% arsenite (cal- .sulated as AS203), is then contacted with low carbon steel. Steel contacted with a solution so treated has been found to be corroded at a rate equivalent to an average pentration of less than 0.001 inch per year.

Example 4.--50 pounds of commercial white arsenic and 75 pounds soda ash are dissolved in 50 gallons tof water at about 95 C. with agitation. The solution so prepared and about 300 pounds of a crude aqueous 33% ammonium thiocyanate solution are added to 45 tons of a solution containing 60% ammonium nitrate, 20%

ammonia, and the remainder chiefly water. The ammoniacal ammonium nitrate solution is then placed in a conventional type tank car for shipment.

Example 5.-1 part of an alkaline arsenite solution prepared as in Example 1 is added to 100 parts of a solution containing 33% urea, 17.7%

combined carbon dioxide and ammonia, 27.8% free ammonia and the remainder chiefly water. The solution so prepared contains 0.1% arsenite (calculated as AS203). This solution is then contacted with low carbon steel which has been found to be corroded thereby at a rate equivalent to a penetration of less than 0.001 inch per year.

Example 6;--1 part of an alkaline arsenite solution, prepared as in Example 1, is added to 100 parts of a solution containing 58% ammonium nitrate, 16% urea, 13% ammonia and the remainder chiefly water. This solution is then contacted with low carbon steel which has been found to be corroded thereby at a rate equivalent to a penetration less than 0.001 inch per year.

We claim:

1. The process of reducing corrosion of a ferrous metal by an ammoniacal solution of an inorganic salt, which comprises incorporating in said solution a trivalent arsenic compound in amount suiiicient to decrease the rate of corrosion of aferrous metal by said solution, and contacting said solution with said ferrous metal. I

2. The process of reducing corrosion of a ferrous metal by an ammoniacal solution of an inorganic salt, which comprises incorporating an arsenite in said solution in amount suificient to decrease the rate of corrosion of a ferrous metal by said solution, and contacting said solution with said ferrous metal.

3. The process of reducing corrosion of a ferrous metal by an ammoniacal solution of. an inorganic salt, which comprises incorporating a sulfide of trivalent arsenic in said solution in amount sufficient to decrease the rate of corrosion of a ferrous metal by said solution, and contacting said solution with saidferrous metal.

4. The process of reducing corrosion of a ferrous metal by an ammoniacal solution of an in-' organic salt, which comprises incorporating in said solution both a trivalent arsenic compound and a compound containing divalent sulfur linked to carbon in amounts suflicient to decrease the rate of corrosion of a ferrous metal by said solution, and contacting said solution with said ferrous metal.

5. The process of reducing corrosion of a ferrous metal by an ammoniacal solution of an inorganic salt, which comprises incorporating in said solution both a trivalent arsenic compound and an alkali metal thiocyanate in amounts suflicient to decrease the rate of corrosion of a ferrous metal by said solution, and contacting said solution with said ferrous metal.

6. The process of reducing corrosion of a ferrous metal by a solution of ammonium nitrate, containing free ammonia, which comprisesv insenic compound and a compound containing divalent sulfur linked to carbon in amounts suflicient to decrease the rate of corrosion of a ferrous metal by 'said solution, and contacting said solution with said ferrous metal.

8. The process of reducing corrosion of a ferrous metal by a solution of ammonium nitrate containing free ammonia, which comprises incorporating in said solution both a trivalent arsenic compound and an alkali metal thiocyanate in amounts suificient to decrease the rate ofcorrosion of a ferrous metal by said solution, and contacting said solution with said ferrous metal.

9. The process of reducing corrosion of a ferrous metal by an ammoniacal solution of ureaand a compound of carbon dioxide and ammonia, which comprises incorporating in said solution a trivalent arsenic compound in amount sumcient to decrease the rate of corrosion of a ferrous metal by said solution, and contacting said solution with said ferrous metal.

10. As a new composition of matter of reduced corrosiveness with respect to ferrous metals, an ammoniacal solution of an inorganic salt containing a trivalent arsenic compound in amount willcient to decrease the rate of corrosion of a ferrous metal by said solution.

11. As a new composition of matter, an ammoniaoal solution of an inorganic salt contain- -1ng a trivalent arsenic compound and a compound containing divalent sulfur linked to carbon in amounts suflicient to decrease the rate of corrosion of a ferrous metal by said solution.

12. As a new composition of matter, an ammoniacal solution of ammonium nitrate containing a trivalent arsenic compound in amount suiilcient to decrease the rate of corrosion of a terrous metalby said solution.

13'.-As anew composition of matter, an ammoniacal solution of ammonium nitrate containingfa trivalent arsenic compound and a compound containing divalent sulfur linked to carbon in amounts suflicient to decrease the rate of corrosion of a ferrous metal by said solution.

' 14. As a new composition of matter, an ammoniacal solution of urea and a compound of ammonia and carbon dioxide containing a trivalent arsenic compound in amount sufficient to decrease the rate of corrosion of a ferrous metal by said solution.

HERMAN A. BEEKHUIS, JR. CHARLES K. LAWRENCE. 

